Method and apparatus for determining and using head parameters in a helical scan recorder

ABSTRACT

Methods of calibrating a helical scan recorder include transporting a storage media past a drum at a controlled linear velocity and recording tracks on the media using a write head. During a read-after-write operation, servo signals recorded on the tracks are read. The servo signals from the tracks are used to determine an axial offset variance of the write head and a read head on the drum. In one mode, calibration is achieved for a first helical scan recorder which does not have a capstan, by installing a drum of the recorder in a second helical scan recorder, using the second recorder to record servo signals on two tracks, reading the servo signals from those two tracks in the second recorder, using the servo signals to determine axial offset variance, and storing a value indicative of axial offset variance in a memory of the first recorder. The axial offset variance is used in a write splice operation and, in one mode, to determine linear velocity of the media.

This is a divisional of application Ser. No. 08/561,155, filed Nov. 21,1995 now U.S. Pat. No. 5,731,921; which is a continuation of applicationSer. No. 08/150,733, filed Nov. 12, 1993 (abandoned).

BACKGROUND

1. Field of Invention

This invention pertains to the operation of a helical scan recorder, andparticularly for determining parameters of the helical scan recorder forenhancing performance thereof.

2. Related Art and Other Considerations

Numerous prior art patents and publications teach recording and readingof information stored in helical stripes (or "tracks") on magneticstorage media (e.g., magnetic tape). In a helical scan arrangement,travelling magnetic tape is at least partially wrapped around a rotatingdrum (or scanner) so that heads (both write head(s) and read head(s))positioned on the drum are contiguous to the drum as the drum isrotated.

One or more write heads on the drum physically record data on the tapein a series of discrete tracks oriented at an angle with respect to thedirection of tape travel. As used herein, track or stripe "pitch" meansa distance between centerlines of two adjacent tracks, the centerlinesof the tracks extending along the direction of head travel and thedistances therebetween being taken perpendicularly to the centerlines.In a dual azimuth system, track pitch equates to the width of a track.The data on the track is formatted, prior to recording on the tape, toprovide sufficient referencing information to enable later recoveryduring readout by one or more read heads.

Examples of helical scan recorders are shown, inter alia, in thefollowing U.S. patents (all of which are incorporated herein byreference):

U.S. Pat. No. 4,835,628 to Hinz et al.

U.S. Pat. No. 4,843,495 to Georgis et al.

U.S. Pat. No. 5,065,261 to Hughes et al.

U.S. Pat. No. 5,068,757 to Hughes et al.

U.S. Pat. No. 5,142,422 to Zook et al.

On a drum of a helical scan recorder, the write head(s) are distancedfrom the read head(s) both by a radial distance and an axial distance(the axial distance being taken along the major axis of the drum). Theseparation of the write head and read head along the major axis of thedrum is herein denoted as the "axial offset" or "axial offset distance".Although a helical scan recorder is intended to be manufactured to havea specification axial offset distance, it generally turns out that thedrum of a helical scan recorder as manufactured has an actual axialoffset distance which varies from the specification axial offsetdistance. As used herein, "axial offset variance" means the differentialbetween (1) a desired (e.g., specification or reference) axial offsetdistance by which a write head is supposed to be separated from a readhead on the drum along the drum axis, and (2) an actual axial offsetdistance by which a write head is actually separated from a read head onthe drum along the drum axis.

To the extent that axial offset variance has been measured in the priorart, such measurements have taken the form of imprecise gauging with theuse of optical measurement devices such as a high power microscope.However, the margin of error of such measurement devices is greater thanthe precision required for use in a helical scan recorder.

Axial offset variance has importance for a number of reasons. Forexample, axial offset variance is a factor which complicates writesplice operations. In a write splice, the recorder must start recordingexactly at a point ("splice location") at which the previous recordinghad stopped. To maximize media usage, the junction of new data to olddata must be seamless, so that track pitches are continuous.

If there is no axial offset variance (e.g., axial offset variance=0),track pitch uniformity can easily be obtained at the write splicelocation. However, even a small axial offset variance (for example, twomicrons) will result in nonuniformity of track pitch at the write splicelocation. Subsequent read operations in the neighborhood of the writesplice location can cause servoing problems, particularly if severalsplices are close together.

Axial offset variance also can be a factor in determining linear tapespeed in certain helical scan recorders, such as a capstanless helicalscan recorder. In this regard, see simultaneously-filed U.S. patentapplication Ser. No. 08/150,726 (attorney docket 1300-135) of Georgisand Zweighaft entitled "Method And Apparatus For Controlling Linear TapeSpeed In A Helical Scan Recorder" (incorporated herein by reference) nowabandoned.

SUMMARY

Methods of calibrating a helical scan recorder include transporting themedia past a drum at a controlled linear velocity and recording trackson the media using a write head during a first angular portion of a drumrevolution. During a second angular portion of a drum revolution, servosignals are read. The servo signals are used to determine an axialoffset variance for the write head and the read head on the drum.

As used herein, "axial offset variance" means a differential between (1)a desired (e.g., specification or reference) axial offset distance bywhich a write head is supposed to be separated from a read head on thedrum along the drum axis, and (2) an actual axial offset distance bywhich a write head is actually separated from a read head on the drumalong the drum axis.

In one mode of the invention wherein the helical scan recorder has afixed-radius, motor-driven capstan and the tracks are read back within180 degrees of recordation, the servo signals obtained during the mediawrite operation are used to obtain a first interim value q(B-K3), inwhich B is a head overlap on a second of two servo-bearing tracks, q isan output voltage per micrometer of track overlap, and K3 is the axialoffset variance. The storage media is then rewound, and the mediatransported past the drum at the controlled linear velocity while amedia read operation is conducted. In the media read operation, tracksrecorded on the tape are read and servo signals recorded thereon areused to obtain further interim values A, B (A being the head overlap ona first of the servo-bearing tracks). The first interim value and thesecond interim value are then used to determine a value indicative ofthe axial offset variance.

Another mode of the invention concerns a helical scan recorder whichdoes not have a capstan, and wherein the tracks are read back at least540 degrees after recordation. In this mode, the drum of the capstanlessrecorder is installed in a test device such as another helical scanrecorder in which media can be transported past the drum at a controlledlinear velocity. Tracks are recorded on the media at the controlledlinear velocity using the write head. Servo signals recorded on twotracks are read back at least 540 degrees later. The servo signals fromthe two tracks are used to determine a value indicative of an axialoffset variance of the write head and the read head on the drum. Thedrum is then removed from the controlled velocity recorder and installedin the capstanless recorder. A value indicative of the axial offsetvariance is stored in a memory of the capstanless helical scan recorder.The stored value indicative of the axial offset variance can then beused to control linear velocity of the storage media in the capstanlessrecorder and to attain uniform track pitch during a write spliceoperation.

In yet another mode of the invention, a fixed length calibration tapehaving length corresponding to a predetermined number of tracks (e.g.,of predetermined calibration information) is installed in the recorder.Then, the recorder records information (e.g., the predeterminedcalibration information) on the installed fixed length calibration tape.A number of tracks actually recorded is then determined, and comparedwith the predetermined number of tracks which perfectly fit on thecalibration tape. The comparison is then used to obtain a parameterrelated to axial offset variance.

A write splice operation according to the invention involves readingtracks previously recorded on a storage media and using a stored valueindicative of the axial offset variance in order to control positioningof the write head. Write head positioning is controlled so that a tracksubsequently recorded at the write splice location will have uniformtrack pitch with tracks previously recorded upstream from the writesplice location. A track of uniform pitch is then recorded at the writesplice location.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments as illustrated in the accompanyingdrawings in which reference characters refer to the same partsthroughout the various views. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention.

FIG. 1A is a schematic view of portions of a capstanless helical scanrecording system according to an embodiment of the invention.

FIG. 1B is a schematic view of portions of a helical scan recordingsystem having a capstan according to an embodiment of the invention.

FIG. 2A is a schematic view of a circumferential surface of a drumutilized in the helical scan recording system of FIG. 1A, the drumappearing as if its circumferential surface were cut and unrolled inplanar fashion.

FIG. 2B is a schematic view of a circumferential surface of a drumutilized in the helical scan recording system of FIG. 1B, the drumappearing as if its circumferential surface were cut and unrolled inplanar fashion.

FIG. 3A is a schematic view of a portion of electronics included in thehelical scan recording system of FIG. 1A.

FIG. 3B is a schematic view of a portion of electronics included in thehelical scan recording system of FIG. 1B.

FIG. 4A is a schematic view of heads traversing dual azimuth helicalstripes in the helical scan recording system of FIG. 1A.

FIGS. 4B and 4C are schematic views of heads in recording and readingoperations, respectively, with a guardband format in the helical scanrecording system of FIG. 1B.

FIG. 4C is a schematic view heads traversing helical stripes having aguardband in the helical scan recording system of FIG. 1B.

FIG. 5 is a flowchart depicting steps executed according to a method ofoperating a helical scan recording system.

FIG. 6 is a flowchart depicting steps executed according to a method ofoperating the helical scan recording system of FIG. 1B.

FIG. 7 is a flowchart depicting steps executed according to a method ofcalibrating a head of a helical scan recording system of FIG. 1A.

FIG. 8 is a flowchart depicting steps executed according to anothermethod of calibrating a head of a helical scan recording system of FIG.1A.

FIG. 9 is a flowchart depicting steps executed according to a method ofexecuting a write splice operation using a helical scan recordingsystem.

FIG. 10 is a schematic view depicting axial offset variance withreference to an exemplary drum of a helical scan recorder.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1A shows portions of a capstanless tape path for a helical scanrecording system generally depicted by reference numeral 20. Inparticular, FIG. 1A shows a magnetic tape 22 (such as an 8 mm magnetictape, for example) having a first end wound around a supply reel 24 anda second end wound around a take-up reel 26. The path traversed by tape22 is defined at least in part by a series of tape guides 28A-28G and arotating scanner or drum 30. Drum 30 has a drum major axis 30X. In alloperations excepting a rewind operation, tape 22 travels from supplyreel 24 to take-up reel 26 in the direction depicted by arrow 31.

Simultaneously-filed U.S. patent application Ser. No. 08/150,730 nowabandoned (attorney docket 1300-134) of Robert J. Miles and JamesZweighaft entitled "Capstanless Helical Drive System" (incorporatedherein by reference) provides a detailed understanding of the particularcapstanless tape path partially depicted in FIG. 1A.

As shown both in FIG. 1A and FIG. 2A, drum 30 has read heads R1 and R2as well as write heads W1 and W2 mounted on the circumference thereof(the exact positioning of which will be described below). Drum 30rotates in the direction depicted by arrow 32. As drum 30 rotates, atany moment a portion of its circumference is in contact with travellingtape. During a recording or write operation, write heads W1 and W2 areperiodically positioned to record "stripes" or "tracks" (such as tracksC2, B1, B2, A1, and A2 shown in FIG. 4A) as heads W1 and W2 move in adirection of head travel (depicted by arrow 34 in FIG. 4A) across tape22. FIG. 4 also depicts track pitch (depicted by arrow 36 in FIG. 4)which (in the illustrated dual azimuth system) is also essentially thewidth of the track (after recordation of neighboring tracks) in adirection. perpendicular to the track centerline (the track centerlineoptimally being parallel to direction 34).

FIG. 4A also illustrates overlap of read head R1 onto adjacent tracks.In particular, read head R1 has an overlap "A" onto adjacent track B2(for reading off-azimuth servo signals recorded on track B2) on anoverlap "B" onto adjacent track C2 (for reading off-azimuth servosignals recorded on track C2).

Helical scan system 20 uses a read-after-write procedure as disclosed insimultaneously-filed U.S. patent application Ser. No. 08/150,726(attorney docket 1300-135) of Georgis and Zweighaft entitled "Method AndApparatus For Controlling Media Linear Speed In A Helical Scan Recorder"(incorporated herein by reference).

FIG. 2A is a schematic depiction of specification-prescribed vertical(e.g., axial) positionings of heads W1, W2, R1 and R2 on drum 30 of theembodiment of FIG. 1A. FIG. 2A shows drum 30 as if its circumferentialsurface were cut and rolled out in planar fashion. In FIG. 2A, acenterline 40 of head W2 is shown per specification to be located adistance 42 above a drum reference surface 44 (e.g., the lower axialsurface of drum 30). FIG. 2A further shows that a lower edge line 46 ofhead W1 per specification is located a distance 0.0153 microns belowlower edge line 41 of head W2; that a lower edge line 48 of head R2 perspecification is located a distance 0.0511 microns below lower edge line41; and, that a lower edge line 50 of head R1 per specification islocated a distance 0.0153 microns below lower edge line 48. The distanceseparating the lower edge lines 41 and 48 along the axis of the drum,known as the "head 2 offset" or "axial offset distance", is depicted bythe distance ΔH.

Placement of heads W1, W2, R1 and R2 on drum 30 results in the formationof tracks as shown in FIG. 4A. In particular, as tape 22 travels pastrotating drum 30, heads W1, W2, R1 and R2 travel in the direction shownby arrow 34. In view of the axial offsets of the heads as described withreference to FIG. 2A, FIG. 4A shows that as write heads W1 and W2 finishrecording their respective tracks A1 and A2 during a first half of adrum revolution, read heads R1 and R2 are almost ready to begin (duringthe second half of the same drum revolution) read-back of tracks B1 andB2, respectively. Tracks B1 and B2 about-to-be-read by heads R1 and R2respectively in FIG. 4A were written during the revolution of drum 30which preceded the revolution during which tracks A1 and A2 wererecorded. Hence, for any track, its read-after-write reading by heads R1and R2 occurs 540 degrees of revolution of drum 30 after the track isrecorded. By now it should be apparent that FIG. 4A illustrates trackshaving the numerical suffix "1" as being written by head W1 andsubsequently read back by head R1. Similarly, tracks having thenumerical suffix "2" are written by head W2 and subsequently read backby head R2.

Although not illustrated herein, it should be understood that servozones are recorded on at least selected tracks. In the illustratedembodiment, servo zones are recorded on tracks written by write head W2.A more detailed understanding of the servo zones utilized by the helicalscan system 20 of FIG. 1A can be gleaned from simultaneously-filed U.S.patent application Ser. No. 08/150,726 (attorney docket 1300-135) ofGeorgis and Zweighaft entitled "Method And Apparatus For ControllingMedia Linear Speed In A Helical Scan Recorder" (incorporated herein byreference).

FIG. 3A shows electronics of the tape drive system 20 of the embodimentof FIG. 1A, including reel motor 50 for rotating supply reel 24 and reelmotor 52 for rotating take-up reel 26 and a reel motor control circuit54. In addition, FIG. 3A shows read signal processing circuitry 60involved in processing signals obtained from read heads R1 and R2; writesignal preparatory circuitry 62; and servo signal processing circuitry64; all under direction of control microprocessor 66.

Details of the read signal processing circuitry 60, write signalpreparatory circuitry 62, and servo signal processing circuitry 64 aremore fully discussed in simultaneously-filed U.S. patent applicationSer. No. 08/150,726 (attorney docket 1300-135) of Georgis and Zweighaftentitled "Method And Apparatus For Controlling Media Linear Speed In AHelical Scan Recorder" (incorporated herein by reference).

As further shown in FIG. 3A, servo signal processing circuitry 64 isconnected to receive the signal motor₋₋ speed_(actual) from take-up reelmotor tachometer 56. Further, servo signal processing circuitry 64receives a signal drum₋₋ speed from a tachometer 120 which is used tomonitor revolutions of drum 30. In addition, servo signal processingcircuitry 64 has access to non-volatile memory 122 in which are storedvarious values and constants, including a constant K3 (axial offsetvariance) here of interest. Also, alternatively, drum₋₋ speed can bepresumed to be a constant and stored in memory 122.

An output terminal of servo signal processing circuitry 64 applies asignal tape₋₋ speed₋₋ correction to reel motor control circuit 54.Examples of structural details of reel motor control circuit 54 areprovided in simultaneously-filed U.S. patent application Ser. No.08/150,727 now U.S. Pat. No. 5,426,355 (attorney docket 1300-146) ofJames Zweighaft entitled "Power-Off Motor Deceleration Control System"as well as in simultaneously-filed U.S. patent application Ser. No.08/546,838 now U.S. Pat. No. 5,892,347 (attorney docket 1300-145) ofJames Zweighaft et al. entitled "High Performance Power Amplifier", bothof which are incorporated herein by reference.

FIG. 1B shows portions of a capstan-utilizing tape path for a helicalscan recording system generally depicted by reference numeral 20'. Indiscussing the embodiment of FIG. 1B and its related drawings FIG. 2Band FIG. 3B, structural members having functions analogous to those ofthe embodiment of FIG. 1A bear analogous (but primed) referencenumerals. In addition to other differences specifically discussedherein, system 20' of FIG. 1B includes a capstan 142' as well as tapeguides 144'. Capstan 142' has an associated capstan tachometer 146'.Capstan 142' imparts a precise linear velocity to tape 22', which linearvelocity is known with reference to output from capstan tachometer 146'.Further, unlike system 20, system 20' uses its servo head S' to readservo zones recorded on tracks written by head W2.

FIG. 2B, in like manner as FIG. 2A, is a schematic depictionspecification-prescribed vertical positioning of heads W1', W2', R1' andR2' on drum 30' including the "axial offset distance" ΔH'. As explainedin simultaneously-filed U.S. patent application Ser. No. 08/150,726(attorney docket 1300-135) of Georgis and Zweighaft entitled "Method AndApparatus For Controlling Media Linear Speed In A Helical Scan Recorder"(incorporated herein by reference), the axial spacings of heads in theembodiment of FIG. 1A differs from the axial spacings of heads in theembodiment of FIG. 1B, with the result that for any track, aread-after-write procedure occurs during subsequent revolutions of drum30 after the track is written (e.g., intervening tracks are recordedbefore a track is read for checking).

Similar to the embodiment of FIG. 1A, the embodiment of FIG. 1B haselectronics which includes write signal preparatory circuitry 62' andread signal processing circuitry 60', as well as a controlmicroprocessor 66'. In addition, electronics of the embodiment of FIG.1B includes a SCSI interface 104'; a buffer manager 106'; and servomotion and control system 152'. Servo motion and control system 152'controls the following sub-systems: drum servo 156'; capstan servo 158';reel control 160'; mechanical control 162'; and, servo filter detector174'.

FIG. 10 illustrates axial offset variance for an exemplary drum of ahelical scan recorder. In other words, FIG. 9 generically represents,for example, drums of helical scan recorders including both the drum 30of the embodiment of FIG. 1A and drum 30' of the embodiment of FIG. 1B.FIG. 10 contrasts the specification-prescribed positions of headsW2_(SPEC) and R2_(SPEC) with the actual positions W2_(ACTUAL) andR2_(ACTUAL) (heads W2_(ACTUAL) and R2_(ACTUAL) being represented bybroken lines). In addition, FIG. 10 contrasts the manufacturingspecification axial offset distance ΔH_(SPEC) and the actual axialoffset distance ΔH_(ACTUAL). As illustrated in FIG. 10,

    ΔH.sub.SPEC -ΔH.sub.ACTUAL =Axial Offset Variance (AOV).

While in FIG. 10 the actual placement of heads W2_(ACTUAL) andR2_(ACTUAL) are shown to be closer together than desired byspecification, other types of variances may be encountered (e.g., headsW2_(ACTUAL) and R2_(ACTUAL) may be further apart than intended).

Consider, for example, FIG. 10 and FIG. 4A in context of the embodimentof FIG. 1A. In the FIG. 1A embodiment, ΔH_(SPEC) =51.10 microns. Toperform a write splice, a helical scan recorder built perfectly tospecification (i.e., AOV=0) would track in a read operation by settingservo amplitudes equal (e.g., 1:1), corresponding to a specificationrequired 4.75 micrometer overlap in each adjacent track (i.e., overlapA=overlap B in FIG. 4A). At the write splice location, thespecification-perfect helical scan recorder would switch to a recordoperation, and create a continuous splice (e.g., the track after thesplice would have the same pitch as the track before the splice).

If, in contrast to a specification-perfect recorder, an actual helicalscan recorder has an axial offset variance of (for example) 2 microns,at the write splice location the first newly recorded track will have awidth (i.e., pitch) that differs from the adjacent tracks by 2 microns.

In the above situation, if the axial offset variance could be reliablymeasured, then compensation for the axial offset variance is madeaccording to the present invention by purposely positioning back by theaxial offset variance. This is done by positioning the tape to createunequal servo amplitudes. For example, if the nominal overlap issupposed to be 4.75 microns and AOV=2 microns, the tape can bepositioned to create unequal servo amplitudes in the ratio of

    (4.75+2)/(4.75):(4.75-2)/(4.75)

or

    1.4:0.6.

Using this criteria, the basic read tracking servo objective becomes:

    q(A-B)/(A+B)=0                                             [Equation 1]

where "A" is overlap of read head R1 on a first adjacent track (e.g.,track B2 in FIG. 4A); "B" is overlap of read head R1 on a secondadjacent track (e.g., track C2 in FIG. 4A); and "q" is servo outputvoltage per micron of track overlap. The determination of "q" is made byfirst measuring the output for full overlap.

Considering now the inclusion of the axial offset variance K3 to theread tracking servo criteria, let

    A=A+K3

    B=B-K3

which, upon substitution into Equation 1, yields

    q(A-B+2*K3)/(A-B)=0                                        [Equation 2]

or

    q(A-B)/(A+B)=-2q*K3/(A+B)                                  [Equation 3]

so that the write head can be positioned correctly for a write spliceoperation by changing the servo criteria by 2q*K3.

As seen below, this servo criteria offset 2q*K3 is used to obtainuniform track pitch when write splicing in both the embodiment of FIG.1A and FIG. 1B. In addition, the value K3 can be computed and used inconnection with tape linear velocity determination (and hence used toensure uniformity of track pitch during even an ordinary write operationin a capstanless system), as explained in simultaneously-filed U.S.patent application Ser. No. 08/150,726 (attorney docket 1300-135) ofGeorgis and Zweighaft entitled "Method And Apparatus For ControllingLinear Tape Speed In A Helical Scan Tape Recorder" (incorporated hereinby reference).

AXIAL OFFSET VARIANCE (AOV) DETERMINATIONS

Axial offset variance (AOV) determinations involve a calibration of thedrum (30 or 30') with respect to specification parameters. Three modesof determinations are described with respective reference to FIG. 6,FIG. 7, and FIG. 8. AOV determinations for the modes of FIG. 6 and FIG.7 have common fundamental steps, illustrated in FIG. 5.

In step 500 of FIG. 5, the tape is transported past thedrum-being-calibrated (the "drum", hereinafter) at a controlledvelocity. For the embodiment of FIG. 1B, such transport can occur in thesystem 30', since system 30' includes capstan 142' whereby the linearvelocity of tape 22' can be controlled. For the embodiment of FIG. 1A,as explained subsequently with respect to FIG. 7, drum 30' is removedand installed in another recorder or comparable device in which tape canbe transported at a controlled velocity.

At step 502, tracks are recorded on the tape using the write head (W1 orW1') during a first angular portion of a drum revolution during a mediawrite operation. At least selected ones of the tracks are servo-bearingtracks which have servo signals recorded thereon. During a secondangular portion of a drum revolution of the tape write operation, servosignals recorded on at least one servo-bearing track are readoff-azimuth (see step 504).

At step 506 the servo signals obtained at step 504 are used to determinethe axial offset distance separating the write head and the read head onthe drum. Then, at step 508 the value indicative of the axial offsetvariance is stored in a memory.

AOV FIRST MODE DETERMINATION

FIG. 6 illustrates steps involved in a first mode AOV determination. Thefirst mode AOV determination is utilized for a helical scan recorder ofthe type of the embodiment of FIG. 1B (e.g., the type which has acapstan and in which recorded tracks are read back within 180 degrees ofrecordation) reading a format which includes a guardband.

At step 602 of FIG. 6, tape 22' is transported past drum 30' at acontrolled linear velocity. As tape 22' is being so transported, at step604 tracks (such as servo-bearing track A2 in FIG. 4B) are recorded onthe media using write head W2' during a first angular portion of a drumrevolution during a media write operation.

At step 606, servo signals recorded on only the most recently-recordedservo-bearing track are read by servo head S' during a second angularportion (e.g., in this case 270 degrees following recordation) of a drumrevolution of the media write operation, due to the location of servohead S' in the embodiment of FIG. 1B (see FIG. 4B). Then, at step 608,servo motion and control system 152' uses the servo signals obtainedduring the read-after-write procedure of the write operation to obtain afirst interim value q(B-K3).

In the above regard, advantageously tape 22' in recorder 20' travels ata controlled linear velocity since recorder 20' has capstan 142'.However, unlike the embodiment of FIG. 1A, servo head S' is notdistanced from head W2' by three track pitches, so head S' reads onlythe "B" overlap. Thus, at step 608 only the interim value q(B-K3) isobtained.

At step 610 servo and motion control system 152' (acting through reelcontrol subsystem 160') causes the tape to be re-wound. Followingrewind, at step 612 tape 22' is transported in the forward directionagain at the controlled velocity (see FIG. 4C). During transport, atstep 614 recorder 30' conducts a tape read operation with tracking servoactivated. In connection with the tape read operation of step 614, atstep 616 servo head S' reads servo signals on two adjacent ones of theguardband-separated servo-bearing tracks.

Having obtained servo amplitudes of two-servo bearing tracks at step 616(i.e., "qA" and "QB") at step 618 servo and motion control system 152'of recorder 20' uses the two servo amplitudes to obtain two furtherinterim values. That is, at step 618 the overlap values A and B aredetermined as shown in FIG. 4C. Knowing the overlap values A and B fromthe step 618 determination, as well as the value q(B-K3) from step 608,at step 620 the axial offset variance K3 can be calculated by servo andmotion control system 152'. Axial offset variance K3 can then be storedin memory for subsequent use during write splice operations.

AOV SECOND MODE DETERMINATION

FIG. 7 illustrates steps involved in a second mode AOV determination.The second mode AOV determination is utilized for a helical scanrecorder of the type of the embodiment of FIG. 1A (e.g., a capstanlesstype in which recorded tracks are read back after more than 180 degreesof recordation).

At step 702 of FIG. 7, drum 30 of recorder 20 is installed in acalibration device (e.g., another helical scan recorder) havingcontrolled linear velocity (e.g., capstan drive recorder). For the sakeof the present discussion, it is assumed that drum 30 of recorder 20 issubstituted for drum 30' in recorder 20' of FIG. 1B, although it shouldbe understood that other calibration devices can instead by employed.

Following the installation of drum 30, at step 704, tape in thecalibration device (e.g., tape 22' in recorder 20') is transported pastdrum 30 at the controlled linear velocity. During thevelocity-controlled transport, at step 706 tracks are recorded by headsW1, W2 on tape 22'. Track recordation occurs during a first angularportion of a drum revolution during a media write operation.

At step 708, during a second angular portion of a drum revolution of themedia write operation (e.g., 540 degrees after recordation), servosignals recorded on two tracks (e.g., tracks C2 and B2 in FIG. 4A) areread, thereby obtaining the "A" and "B" overlaps (see FIG. 4A). At step710, control microprocessor 66' uses the "A" and "B" values, as well asthe "q" value discussed above, to determine axial offset distance inaccordance with Equation 3.

At step 712 drum 30' is removed from the calibration device (e.g.,recorder 20') and installed in capstanless recorder 20. Accompanyinginstallation of drum 30 in recorder 20, at step 714 the axial offsetvariance K3 obtained from step 710 is loaded into memory 122 of recorder30 (see FIG. 3A) for subsequent use by servo signal processing circuitry62. Axial offset variance K3 has various uses, including (as indicatedby step 716) being a factor in a determination of tape linear velocityin a manner understood with reference to simultaneously-filed U.S.patent application Ser. No. 08/150,726 (attorney docket 1300-135) ofGeorgis and Zweighaft entitled "Method And Apparatus For ControllingMedia Linear Speed In A Helical Scan Recorder" (incorporated herein byreference). Also, as indicated by step 718, axial offset variance K3 isused to obtain uniform track pitch during a write splice operation(understood with reference to the ensuing discussion of FIG. 9.

AOV THIRD MODE DETERMINATION

FIG. 8 illustrates steps involved in a third mode AOV determination. Thethird mode AOV determination is illustrated for a helical scan recorderof the type of the embodiment of FIG. 1A (e.g., a capstanless type inwhich recorded tracks are read back after more than 180 degrees ofrecordation).

At 802 a fixed length tape is installed in system 30. The fixed lengthtape is of a length sufficient to have recorded thereon a predeterminednumber of tracks followed by an end of tape marker. For example, thefixed length tape may be of a length corresponding to 1000 tracks.

At step 804, information such as predetermined calibration informationis loaded into write preparatory circuitry 62 of recorder 30 andrecorded on the fixed length tape until end of tape is encountered. Theservo signals are read back during the write process as described in AOVSecond Mode Determination, supra. The linear tape speed is modified toset the readback signals equal as described in simultaneously-filed U.S.patent application Ser. No. 08/150,726 (attorney docket 1300-135) ofGeorgis and Zweighaft entitled "Method And Apparatus For ControllingLinear Tape Speed In A Helical Scan Recorder" (incorporated herein byreference).

At step 806 control microprocessor 66 (or, alternatively, servo signalprocessing circuitry 62) determines the number of tracks actuallyrecorded in attempting to record the predetermined calibrationinformation on the fixed length tape.

At step 808 control microprocessor 66 (or, alternatively, servo signalprocessing circuitry 62) compares the number of tracks actually recordedwith the predetermined number of tracks which perfectly fit on the fixedlength tape (e.g., 1000 in the present example). If, for example, atstep 806 it were determined that only 980 tracks were recorded, it canthen be surmised that the linear tape velocity of the fixed length tapewas run 2% too fast in recorder 30. Alternatively, if 1020 tracks wererecorded, the fixed length tape was run 2% too slow in recorder 30.

At step 810 control microprocessor 66 (or, alternatively, servo signalprocessing circuitry 62) uses the comparison of step 808 to obtain axialoffset variance. In this regard, in one mode of executing step 810, alook-up table stored in a memory (such as memory 122) is consulted. Thelook-up table can have stored therein information such as thatillustrated in Table 1 (Table 1 being applicable to an embodiment havingnominal 15.5 micron track pitch). From Table 1 it can be concluded thatthe tracking error for a 2% too fast speed error is -16.87%, whichdefines 2q*K3/(A+B). From this value, axial offset variance K3 isdetermined.

WRITE SPLICE OPERATION

FIG. 9 shows basic steps applicable to a write splice operation foreither the helical scan recorder system 20 of FIG. 1A or the helicalscan recorder system 20' of FIG. 1B. At step 902, tracks previouslyrecorded on the tape (22 or 22') are read up to the write splicelocation. In connection with previous track reading at step 902,however, as indicated by step 904 the servoing scheme is altered inanticipation of a write splice. In particular, the servo controller (62or 152') uses the stored value indicative of the axial offset variancein order to control positioning of the heads so that, including thewrite heads, so that a track subsequently recorded at the write splicelocation will have uniform track pitch with tracks previously recordedupstream from the write splice location. In this regard, the servoingcriteria is modified by setting the criteria back by the amount 2q*K3.Step 906 reflects recordation of at least one new track at the writesplice location (it being understood that most likely many new trackswill be recorded).

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various alterations in form and detail maybe made therein without departing from the spirit and scope of theinvention. Moreover, although the particular media illustrated herein ismagnetic tape, the invention is not limited thereto but can be used withother media employed in a helical scan environment.

                  TABLE 1                                                         ______________________________________                                                       R1                                                             Actual Written Center  R1 head                                                                             R1 head     Track-                               Tape   Track   line    overlap                                                                             overlap                                                                             Speed ing                                  Speed  Width   Error   -     +     Error Error                                ______________________________________                                        31.56  14.73   1.94    7.08  3.20  -5.00%                                                                              37.71%                               31.73  14.80   1.74    6.84  3.36  -4.50%                                                                              34.20%                               31.90  14.88   1.55    6.61  3.51  -4.00%                                                                              30.63%                               32.06  14.96   1.36    6.38  3.67  -3.50%                                                                              27.01%                               32.23  15.04   1.16    6.15  3.82  -3.00%                                                                              23.33%                               32.40  15.11   0.97    5.91  3.98  -2.50%                                                                              19.60%                               32.56  15.19   0.77    5.68  4.13  -2.00%                                                                              15.50%                               32.73  15.27   0.58    5.45  4.29  -1.50%                                                                              11.94%                               32.89  15.35   0.39    5.22  4.44  -1.00%                                                                              8.03%                                33.06  15.42   0.19    4.98  4.59  -0.50%                                                                              4.05%                                33.23  15.50   0.00    4.75  4.75  0.00% 0.00%                                33.39  15.58   -0.19   4.52  4.91  0.50% -4.11%                               33.56  15.66   -0.39   4.29  5.06  1.00% -8.29%                               33.72  15.73   -0.58   4.05  5.21  1.50% -12.54%                              33.89  15.81   -0.77   3.82  5.37  2.00% -16.87%                              34.06  15.89   -0.97   3.59  5.52  2.50% -21.26%                              34.22  15.97   -1.16   3.35  5.68  3.00% -25.73%                              34.39  16.04   -1.36   3.12  5.84  3.50% -30.28%                              34.55  16.12   -1.55   2.89  5.99  4.00% -34.91%                              34.72  16.20   -1.74   2.66  6.15  4.50% -39.62%                              34.89  16.28   -1.94   2.42  6.30  5.00% -44.41%                              ______________________________________                                    

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of conducting awrite splice operation with a helical scan recorder which recordsinformation in helical tracks upon a storage media, the helical scanrecorder having a rotating drum upon which a write head and a read headare mounted, the write head and read head being separated on the drum byan actual axial offset distance which differs from a reference axialoffset distance by an axial offset variance, the write splice operationto occur at a write splice location, the method comprising:readingtracks previously recorded on the storage media, including servoinformation recorded on the tracks up to the write splice location;determining a value indicative of the axial offset variance; storing thevalue indicative of the axial offset variance in a memory; using thestored value indicative of the axial offset variance in order to controlpositioning of the write head so that a track subsequently recorded atthe write splice location will have uniform track pitch with trackspreviously recorded upstream from the write splice location; recordingat least one track at the write splice location.
 2. The method of claim1, wherein the value indicative of the axial offset variance isdetermined by:transporting the media past the drum at a controlledlinear velocity; recording tracks on the media using the write headduring a first angular portion of a drum revolution during a media writeoperation, at least selected ones of the tracks being servo-bearingtracks which have a servo signal recorded thereon; reading, during asecond angular portion of a drum revolution of the media writeoperation, servo signals recorded on at least one servo-bearing track;using the servo signals to determine the axial offset variance of thewrite head and the read head on the drum.
 3. The method of claim 1,wherein the value indicative of the axial offset variance is determinedby:(a) transporting the media past the drum at a controlled linearvelocity; (b) recording tracks on the media using the write head duringa first angular portion of a drum revolution during a media writeoperation, selected tracks being servo-bearing tracks which have a servosignal recorded thereon; (c) reading, during a second angular portion ofa drum revolution of the media write operation, servo signals recordedon at least one servo-bearing track; (d) using the servo signals readduring the media write operation of step (c) to obtain a first interimvalue; then (e) rewinding the storage media; (f) transporting the mediapast the drum at the controlled linear velocity; then (g) conducting amedia read operation by reading tracks recorded on the tape as the mediais transported at the controlled linear velocity; (h) reading, duringthe media read operation, servo signals recorded on two adjacent ones ofthe servo-bearing tracks; (i) using the servo signals read from the twoadjacent ones of the servo-bearing tracks during the media readoperation to obtain at least a second interim value; then (j) using thefirst interim value and the second interim value to determine a valueindicative of the axial offset variance.
 4. The method of claim 1,wherein the value indicative of the axial offset variance is determinedby:installing the drum in a test device in which media can betransported past the drum at a controlled linear velocity, and in thetest device; transporting the media past a drum at the controlled linearvelocity; recording tracks on the media using the write head during afirst angular portion of a drum revolution during a media writeoperation, at least selected tracks having a servo signal recordedthereon; reading, during a second angular portion of a drum revolutionof the media write operation, servo signals recorded on two tracks;using the servo signals from the two tracks to determine a valueindicative of an axial offset variance of the write head and the readhead on the drum; then removing the drum from the test device andinstalling the drum in a helical scan recorder; storing the valueindicative of the axial offset distance in a memory of the helical scanrecorder.
 5. The method of claim 1, wherein a value indicative of theaxial offset variance is obtained from a head parameter determinedby:(a) installing in the helical scan recorder a fixed lengthcalibration media, the fixed length being related to a predeterminednumber of tracks recordable thereon; (b) recording information on theinstalled fixed length calibration tape; (c) determining a number oftracks actually recorded at step (b); (d) comparing number of tracksactually recorded at step (b) with the predetermined number of tracks;and (e) using the comparison of step (d) to obtain a parameterconcerning the drum.